A short discussion of fundamentals is beneficial to an understanding of the prior art. A gas laser generates a monochromatic light beam having a very narrow frequency bandwidth. Thus, the exact wavelength of the light may be accurately determined. The frequency of a generated laser beam will fall somewhere within a finite gain profile of the spectral line bandwidth of the lasing material, which is defined by the gas species and the nature of the spectral broadening mechanism (e.g., Doppler broadening, pressure broadening, etc.)
A Doppler broadened profile of the 633 nm laser transition in helium-neon is graphically shown in FIG. 1, with light frequency plotted relative to intensity for a light beam emitted from an excited He--Ne lasing material. The bell-shaped curve indicates the range of frequencies within which laser emission can be obtained. The frequency of the He--Ne laser beam will fall somewhere within this spectral bandwidth, as indicated by the line F.sub.1, though the exact position depends critically on the instantaneous length of the laser resonating chamber. The Doppler profile of the He--Ne spectral line typically has a width of 1500 MHz, though the instantaneous bandwidth of generated laser beam bandwidth, as indicated, is very narrow. The resultant laser beam frequency may thus vary by approximately one in 10.sup.6, which allows a very large range of corresponding light wavelengths, unless a means is applied to control the frequency of the laser beam.
The frequency of the laser beam can be controlled by modulating the distance between the reflecting surfaces at the ends of the lasing chamber. This is normally accomplished by changing the length of the chamber through electro-strictive and/or thermal length varying elements. Prior art has taught the use of a piezoelectric element mounted into or on the outside of the tubular wall of the lasing chamber, which constricts when control voltage is applied to provide immediate changes in cavity length. Heating elements or coils have been applied to the exterior of the tubular wall of the chamber to induce thermally actuated changes in length which, though slower in reaction, provide a greater range of adjustment.
Through use of these techniques, the narrow bandwidth frequency of the laser beam may be positioned, or tuned, to a desired value within the spectral profile and stabilized at a desired frequency for a long period of time.
When a gas laser is subjected to an axial magnetic field circular birefringence induced in the active lasing material by the magnetic field produces the Zeeman effect. This results in the formation of two individual component modes in the laser beam having opposing right and left circular polarizations, and differing slightly in frequency. The frequency difference between the component modes is represented as .DELTA.f in FIG. 2, which difference may vary in the order of 100 to 1500 KHz depending upon the strength of the magnetic field applied. A pair of component modes are indicated in FIG. 2 as F.sub.1R and F.sub.1L, which display a difference in frequency of .DELTA.f.sub.1. The Zeeman effect results in a split of the spectral line profile into two components, indicated by the shifted "Doppler" profiles to the right and left of the original profile (indicated in dotted line). Each of the component modes of the laser beam are likely to have a different light intensity, depending upon their position within the expanded spectral profile. This is indicated in FIG. 2 by the component F.sub.1R having a greater intensity than the component F.sub.1L.
The value of .DELTA.f.sub.1 depends on magnetic field strength and on the location of the mean frequency within the "Doppler" profile.
The difference in frequency .DELTA.f reaches a minimum as the component modes becomes symmetrically positioned about the line center of the original spectral profile (f.sub.0). Minimizing the frequency difference .DELTA.f between component modes provides means of indicating the laser is operating instability.
When the component modes of the laser beam are heterodyned, the resultant wave is representative of the difference in frequencies .DELTA.f between the component modes by exhibiting a characteristic beat. Heterodyning obtains a resultant signal having a characteristic beat which corresponds to the difference in the frequencies of the mixed waves. The resultant beat signal has a first frequency which is the average of the frequencies of the mixed waves, and has an amplitude which oscillates in magnitude or beats with time at a much slower second frequency. The second frequency or number of beats occurring per second is equal to the difference in frequencies of the combined waves as described above. This is often referred to as the Zeeman beat signal of the component modes. The frequency difference between component modes can be easily determined by digitally processing the beat count per unit time of the Zeeman beat signal. Thus the Zeeman beat signal provides a clear indication of frequency difference between the component modes, which can be used to control the frequency stability of the magnetically influenced laser.
These properties and phenomena of lasers influenced by a magnetic field have been taught by a number of published studies.
A laser permits the use of a direct measurement method of interferometry in which the light wavelength of the laser beam serves as a standard unit of length for the measurement. The laser is advantageous due to a characteristic narrow frequency bandwidth, sharp focus and high intensity of its output light beam, which provides an identifiable and accurately measurable light wavelength. The use of a laser in interferometry permits accuracy in length measurement to within fractions of a micron and permits simplistic digital processing techniques to perform such measurement.
For measurements of length by an interferometric technique, in which the wavelength of the laser beam is used as a standard unit of measure, it is necessary to stabilize the laser frequency at a particular predictable and constant value in the Doppler expanded spectral profile with high accuracy of at least one in 10.sup.7, to enable accurate measurement.
The use of a laser exhibiting a Zeeman split beam is particularly advantageous due to its characteristic beat frequency, which is easily measurable.
Prior art teachings have shown a number of systems for controlling the frequency stability of an output laser beam. For instance, Lang and Bouwhuis have taught a means of tuning a laser by inducing electro-strictive and thermal changes in the length of the lasing inducing chamber. The lasing chamber length is adjusted to stabilize the median frequency of the laser beam (the median frequency being the average frequency between the Zeeman split component modes) at the known spectral line which is characteristic of the lasing material. Frequency stabilization is accomplished by measuring the intensity difference between the component modes and using the intensity difference measurement to generate a control signal to correct chamber length. The intensities of each of the component modes are alternately measured by an intensity sensitive photodetector. This is accomplished through selective transmittance of each of the component mode beams through an electro-optical crystal whose birefringence is modulated by an a.c. signal to alternately pass one or the other of the component frequencies for intensity measurement by the photodetector. This results in an alternating signal which is compared with the a.c. signal applied to a crystal to obtain a measurement of the difference in intensity of each of the component modes. A control signal is generated responsive to the intensity difference, which controls voltage applied to the chamber length tuning elements of the laser.
The chamber length is adjusted to equalize the intensities of the component modes, thus positioning each of the component modes symmetrically about the line center of the spectral profile, as discussed earlier. This also obtains a minimum frequency difference .DELTA.V between component modes.
U.S. Pat. No. 3,534,292 of Cutler discloses in a system for modulating the length of the lasing chamber, through use of a piezoelectric element, to produce a frequency difference .DELTA.V between component modes which is continually modulated. A signal representing the modulated frequency difference is supplied to a frequency discriminator which converts the signal to one having an a.c. and d.c. component. The a.c. component is used to control the range of modulation of the lasing chamber, and thus .DELTA.V through use of a phase shift circuit. The a.c. component is detected to provide an error correction signal coupled to the piezoelectric element to stabilize the laser component frequencies about the line center of the spectral profile of the lasing medium. The frequency difference between component modes is controlled by a differential amplifier which references a d.c. reference voltage supplied to a differential amplifier which generates a signal to control the strength of the magnetic field applied to the lasing chamber.
Morris and Ferguson have discussed a method of frequency stabilization for a laser influenced by a magnetic field, which consists of heterodyning the component mode frequencies and feeding the heterodyne signal to a comparator to obtain a frequency-to-voltage conversion signal which is applied to an integrator. The signal received from the integrator determines a load to be applied to a heating element wound around the laser cavity wall which introduces a thermal adjustment to the wall of the lasing chamber. This system is used to control the position of the component mode frequencies within the spectral profile.
Hall in U.S. patent application Ser. No. 300,363 filed Sept. 1981 teaches a method of stabilizing the frequency of a laser which comprises obtaining an error signal through dithering (frequency modulating) the Zeeman split component modes within the spectral range and measuring the difference caused in the component mode frequencies by each dither. An up-down counting technique is used to measure change in frequency responding to each direction of the dither, which measurements are compared to determine equal change. Laser cavity length is adjusted to obtain a minimum frequency change of the component modes throughout the dither cycle. Obtaining an equal frequency difference centrally positions the component modes symmetrically about the line center of the spectral profile due to the parabolic function of frequency difference change relative to Doppler expanded spectral profile. The laser cavity length is servo controlled to maintain the frequency difference between component modes at a minimum value by continually applying the dither and counting the relative change in frequency difference between component modes in each direction of its cycle. The counts are maintained equal and opposite in sign.
The Hewlett-Packard Company, Inc. manufactures a gas laser utilizing the Zeeman effect to obtain two component frequencies, which is identified as Model 5525A. By a method similar to that of Lang and Bouwhis the laser is tuned to the line center of the spectral profile through control of a piezoelectric wafer which forms part of the wall of the laser cavity. The piezoelectric element is controlled by an electronic servo loop. The control loop obtains measurement of the intensities of each of the output frequencies of the component modes separately and compares them to drive the servo to maintain the intensities equal through adjustment to the length of the laser cavity. Equating intensities centers the component frequencies about the line center of the spectral profile. Thus, the frequency of each of the Zeeman split component modes is controlled to closely approach the frequency of the line center of the lasing material spectrum profile and the frequency difference is maintained at a minimum to allow accurate prediction of the frequency difference.
Each of the described means and methods of stabilizing the frequency of a Zeeman influenced laser beam has failed to provide an accurate control of the difference in frequency .DELTA.V between component modes, or, in other words, the beat signal exhibited by their heterodyne mixing. This is a highly important parameter in obtaining an accurate inferometric system which uses the beat frequency as representative of a standard unit of measure. The Lang and Bouwhis, and Hewlett-Packard, stabilization systems adjust the component mode frequencies symmetrically about the line center of the spectral profile, to a point where the difference frequency between the component modes is at a minimum and most predictable. In these methods it is the frequency of the component modes which is controlled within the spectral profile to obtain a predictable difference frequency between them. This is accomplished by a measurement function (i.e., of intensity) for each of the component modes which introduces a third tier of error possibility in the control system. The measurements, and companion operations require more complicated circuitry for the control system. Since the actual value of the difference in frequency is not controlled, the frequency difference may not be held constant and may change from one laser to the next in a production run. Furthermore, the value of the frequency difference may be perturbed by external magnetic fields even though the component mode frequencies are being controlled, resulting in different frequency differences in different environments.
Nor is the frequency difference .DELTA.V directly controlled by Hall or Cutler. The method described by Hall sought to minimize the frequency difference between component modes. The dither or frequency modulation technique used to obtain a measurement of the frequency difference and determine when a minimum is reached requires a complicated and expensive servo system. Furthermore, the dithers from an optimum position constantly cause a slight change in frequency difference, which derogatorily affects stability. Also, the necessity of performing a dither lowers frequency response. The continual modulation of the length of the lasing chamber and thus the difference frequency taught by Cutler clearly affects the ability of the servo system to accurately stabilize the frequency difference. This technique approximates that of Hall in applying a dither to continually change the frequencies to obtain a comparison value indicative of a minimum frequency difference. The reference signals provided in Cutler do no more determine median values for the loads applied to the length adjusting elements and the magnetic field coil.
The Morris and Ferguson system admittedly has an observed frequency difference variation of 200 Hz which clearly indicates that the ability of the system to control the frequency difference is limited. Furthermore, there is no indication given as to what the heterodyned beat signal is compared with to obtain a frequency-to-voltage conversion. It would seem that the teaching presented merely indicates that the frequency difference signal can be integrated to obtain a control signal adapted to tune the length of the lasing chamber, which is clearly known in the art.
Each of the above-referenced teachings has obtained control of the frequency difference between component modes of a Zeeman split laser by indirect techniques which position the individual component mode frequencies symmetrically within the Doppler expanded profile of the spectral line of the lasing material, to obtain a predictable value. A clear need remains for a stabilization control which can accurately and directly determine the frequency difference, or beat signal, produced by a laser influenced by a magnetic field. Accuracy in stabilizing the frequency difference is clearly advantageous in interferometric techniques for measurement where the frequency difference or beat signal supplies the basic unit, a wavelength of the laser beam, used for measurement.